Developer cartridges that are detachably mounted in an image-forming device, such as a color printer, are well known in the art. One such developer cartridge includes a developing roller for carrying a developer, and a supply roller for supplying developer to the developing roller. The developer cartridge also includes an input gear; and a developing roller drive gear and a supply roller drive gear engaged with the input gear. By transmitting a drive force to the input gear from a source outside the cartridge, the developing roller and supply roller are driven to rotate through the developing roller drive gear and supply roller drive gear.
Since the developing roller and supply roller are in sliding contact with each other, the ratio of circumferential speeds of supply roller to developing roller has an effect on image quality, the lifespan of the developer, and the like. Specifically, a high ratio of circumferential speeds (hereinafter, the “ratio of circumferential speeds” will refer to the ratio of the circumferential speed of the supply roller to the circumferential speed of the developing roller) can improve image quality since the supply roller has a heightened ability to scrape off residual developer from the surface of the developing roller. When the ratio of circumferential speeds is low, the life of the developer can be increased since there is less friction on the developer between the developing roller and supply roller.